In the prior art there has been considerable effort expended in extending the spectral band-width, or wavelength range, of a broad class of optical metrology instruments. Non-contact, optical measurements are heavily utilized in the optics, optical communications and semiconductor industries. The instruments are used in the evaluation and characterization of samples that can include spatially non-uniform distributions of a broad class of materials including, insulators, semiconductors and metals; consequently, the optical properties of these samples can vary markedly with wavelength. Hence, in general, a much greater wealth of information can be extracted from broad-band spectroscopic measurements than can be obtained from measurements made over a narrow spectral range. One approach to implementing broad-band spectroscopic measurements is set forth in WO 99/02970 which is incorporated herein by reference.
Additionally, the drive to reduce minimum feature size or critical dimensions in semiconductor manufacturing has forced lithography systems to employ ever-decreasing exposure wavelengths. At present, leading-edge industrial lithography systems operate in the DUV over a narrow wavelength region of approximately 193 nm. In the near future systems will operate in the vacuum ultra-violet at a wavelength of 157 nm. The next generation lithography systems are expected to operate in the extreme ultraviolet at 13 nm. Consequently, to keep pace with the manufacturing technology, optical metrology system manufacturers must continually extend the spectral range of measurement systems to shorter measurement wavelengths. Ideally, the wavelength extension results in an increase the measurement band-width of the metrology system.
In the prior art, optical metrology systems that operate over the spectral range spanning the DUV and visible utilize two lamps; a Deuterium lamp for spectroscopic measurements between 190 and 400 nm, and a Xenon lamp for measurements between 400 nm and 800 nm. There are several benefits to this approach. First, the design of the individual lamps can be optimized to tailor the spectral emission over a narrow spectral range simplifying the lamp design. Second, the use of two lamps insures the ability to perform measurements using illumination with the desired bandwidth; avoiding harmonic contamination of the illumination and minimizing unnecessary UV exposure of both the sample and the system optics. For example, UV illumination is admitted to the measurement beam path only when UV measurements are to be performed. This latter feature is most desirable since it minimizes the potential for UV damage to the sample and solarization of the system optics and samples.
In a typical two lamp arrangement each lamp is located in its own housing, and the lamps are arranged so that a part of the illumination from both lamps can be directed along a common optical path to illuminate the sample to be measured. This generally has involved the use beam-splitters, optical elements that partially reflect and partially transmit the incident beam to combine the beams from the two sources. The use of beam splitters necessarily reduces the light intensity reaching the sample since a portion of the useful optical output of each lamp is discarded. Reduced intensity at the sample can result in lower measurement accuracy, increased measurement time and lower system throughput. These effects may be mitigated by using higher output lamps to compensate for the system losses. However, higher output lamps are more expensive and generate higher thermal loads increasing both the capital and operational costs of the metrology system.
Accordingly it would be desirable to provide an illumination system employing two or more lamps that avoids the use of beam splitters yet permits combination of the lamp outputs with the combined output directed along a common optical path. Such an illumination system would be useful to combine the light output of two or more lamps having different spectral output characteristics. Alternatively, where it is desired to increase the light intensity reaching the sample, the subject illumination system can also be used to combine the output of two or more lamps having substantially similar spectral output characteristics.